Open-Path spectroscopic analyzers are used to provide quantitative information on a chemical mixture of gases and aerosol that propagate from emission sources. Detailed information on the emissions can sometimes be obtained from the sources using multiple open-paths that may be scanned by a single Open-path spectrometer. Detection of the chemical mixture of gases and aerosol includes data acquisition of sample spectrum, data reduction and selection of a background spectrum. There are three data acquisition scenarios. The first discloses a series of electromagnetic spectra S(λ) (intensity S vs. wavelength λ) acquired sequentially over a defined path length (extractive or open path). The second discloses a series of electromagnetic spectra acquired sequentially over a spatially adjacent direction. This can be achieved for example with passive infrared or passive ultraviolet spectrometer pointing to the sky or to a blocking mountain range. The third scenario discloses a series of electromagnetic spectra acquired sequentially over the same direction from a moving platform. Examples: solar occultation flux, mobile sky looking spectroscopy, and airborne down looking spectroscopy.
There are three steps of data reduction for the input spectrum (absorption, emission spectrum, or extinction with particle presence) into the quantification algorithm. The first is acquisition and data reduction of the single beam sample spectrum S(λ) and the second step is the selection of the single beam background spectrum S0(λ). The transmission is the ratio
  (      S          S      0        )used to determine absorbance, emission or extinction. In the third step, the transmission is used for generation of a double beam (DB) value and subsequently for determining the quantity of an air contaminant in a plume. The absorbance spectrum is defined as the negative logarithm of S divided by S0, represented by the equation
  A  =      -                            log          10                ⁡                  (                      S                          S              0                                )                    .      Extinction is defined by the sum of absorption and scattering. The emission spectrum is defined as the logarithm of S divided by S0, represented by the equation
  E  =                    log        10            ⁡              (                  S                      S            0                          )              .  